Projection optical system, production method thereof, and projection exposure apparatus using it

ABSTRACT

In a projection optical system having at least two silica glass optical members, a birefringence value is measured at each of points in a plane normal to the optical axis with the center at an intersection of each optical member with the optical axis, a distribution of signed birefringence values in each optical member is obtained by assigning a positive sign to each birefringence value when a direction of the fast axis thereof is a radial direction to the intersection with the optical axis and assigning a negative sign to each birefringence value when the direction of the fast axis thereof is normal to the radial direction, and the optical members are combined with each other so as to satisfy such a placement condition that a signed birefringence characteristic value of the entire projection optical system calculated based on the distributions of signed birefringence values is between −0.5 and +0.5 nm/cm both inclusive. This permits minimization of influence from nonuniform distribution of birefringence values in the optical members on the imaging performance of the projection optical system or on the resolution of projection exposure apparatus and in turn enables provision of the projection optical system with high imaging performance, a production method thereof, and the projection exposure apparatus capable of achieving high resolution.

RELATED APPLICATIONS

This is a Divisional application of prior pending application Ser. No.09/654,269 filed Sep. 1, 2000 now U.S. Pat. No. 6,366,404 which is aContinuation-in-Part application of pending Application No.PCT/JP00/00027, filed Jan. 6, 2000. The entire disclosure of the priorapplication(s) is hereby incorporated by reference herein in itsentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a projection optical system, aproduction method thereof, and a projection exposure apparatus using it.More particularly, the invention concerns a projection optical systemused for transferring a predetermined mask pattern onto a substrate byuse of a light source of an ultraviolet region suitable forsemiconductor fabrication, a production method thereof, and a projectionexposure apparatus using it.

2. Related Background Art

An example of the conventional projection exposure apparatus forsemiconductor fabrication is one having the structure as illustrated inFIG. 16A and FIG. 16B.

Specifically, the projection exposure apparatus 800 illustrated in FIG.16A is constructed in such structure that rays from a light source 501such as a mercury-arc lamp or the like are collected by an ellipsoidalmirror 502 and that thereafter they are converted into a bundle ofparallel rays by a collimator lens 503. Then this parallel beam travelsthrough a fly's eye lens 504, which is an aggregate of optical elements504 a of a square section as illustrated in FIG. 16B, to form aplurality of light source images on the exit side thereof. An aperturestop 505 having a circular aperture is disposed at the position of thelight source images. Beams from the plurality of light source images arecondensed by a condenser lens 506 to uniformly illuminate a reticle R asan object to be illuminated, in superimposed fashion.

A pattern on the reticle R kept under uniform illumination by theillumination optical system in this way is projected onto a wafer Wcoated with a resist, by a projection optical system 507 consisting of aplurality of lenses. This wafer W is mounted on a wafer stage WS, whichis arranged to move two-dimensionally, and the projection exposureapparatus 800 of FIG. 16A is designed to perform exposure by theso-called step-and-repeat method in which the wafer stage issuccessively moved two-dimensionally in order to implement exposure in anext shot area after completion of exposure in one shot area on thewafer.

In recent years proposals have been made about such scanning exposuremethods that a rectangular or arcuate beam was radiated onto the reticleR and that the reticle R and wafer W located in conjugate with eachother with respect to the projection optical system 507 were moved in afixed direction whereby the pattern of the reticle R could betransferred in high throughput onto the wafer W.

In the projection exposure apparatus in either of the above methods, itis desirable that optical members used in their optical systems havehigh transmittance at the wavelength of the light source used. Thereason is as follows: the optical systems of the projection exposureapparatus are constructed of a combination of many optical members; evenif optical loss per lens is little the total transmittance will bedecreased greatly when the optical loss is added up by the number ofoptical members used. If an optical member inferior in transmittancewere used, it would absorb the exposure light to increase thetemperature of the optical member itself and thus cause heterogeneity ofrefractive index and, in turn, local thermal expansion of the opticalmember would deform polished surfaces. This would degrade the opticalperformance.

On the other hand, the projection optical systems are required to havehigh homogeneity of refractive index of the optical members in order toachieve a finer and sharper projection exposure pattern. The reason isthat variations in refractive index will cause a lead and a lag of lightand this will greatly affect the imaging performance of the projectionoptical system.

Thus silica glass or calcium fluoride crystals high in transmittance inthe ultraviolet region and excellent in homogeneity are generally usedas materials for the optical members used in the optical systems of theprojection exposure apparatus in the ultraviolet region (not more thanthe wavelength of 400 nm).

Proposals of decreasing the wavelength of the light source have beenmade recently in order to transfer a finer mask pattern image onto thewafer surface, that is, in order to enhance the resolution. For example,decrease of wavelength into a shorter range is under way from the g-line(436 nm) and the i-line (365 nm), which have been used heretofore, toKrF (248 nm) and ArF (193 nm) excimer lasers.

In the projection exposure using such shorter-wavelength excimer lasers,since the purpose is to obtain the finer mask pattern, the materialsused are those with higher performance as to the homogeneity oftransmittance and refractive index.

SUMMARY OF THE INVENTION

With use of such materials having the high homogeneity of transmittanceand refractive index, however, there were cases wherein desiredresolution was not obtained after the optical system was assembled incombination of plural materials.

An object of the present invention is, therefore, to provide aprojection optical system having high imaging performance, a productionmethod thereof, and a projection exposure apparatus capable of achievinghigh resolution.

The inventors have conducted intensive and extensive research in orderto accomplish the above object and first found out that birefringence ofthe materials of the optical members greatly affected the imagingperformance of the projection optical system and the resolution of theprojection exposure apparatus. Then the inventors discovered that theimaging performance close to designed performance of the projectionoptical system and the resolution close to designed performance of theprojection exposure apparatus were attained if the magnitude ofbirefringence, i.e., birefringence values (absolute values) of thematerials of the optical members were not more than 2 nm/cm and ifdistribution of birefringence was symmetric with respect to the centerin the optical members, and disclosed it in Japanese Patent ApplicationLaid-Open No. 8-107060.

With increase in demands for much higher resolution of the projectionexposure apparatus, however, there were cases wherein satisfactoryimaging performance of the projection optical system and satisfactoryresolution of the projection exposure apparatus were not attained evenwith employment of the above conventional design concept if light of theshorter wavelength was used as a light source or if an optical memberhaving a large diameter and a large thickness was used.

Thus the inventors have conducted further research and, as a result,discovered that the cause of failing to obtain the projection opticalsystem and projection exposure apparatus of desired optical performanceeven with use of the optical members having good homogeneity oftransmittance and refractive index was that distribution states ofbirefringence of the optical members differed among the optical members,the different birefringence distributions were added up in the overalloptical system where the projection optical system was constructed incombination of a plurality of optical members, and this resulted indisturbing the wavefront of the light in the overall optical system,thereby greatly affecting the imaging performance of the projectionoptical system and the resolution of the projection exposure apparatus.

Describing the above in more detail, the conventional ways of evaluatingthe birefringence of optical members were nothing but arguments aboutwhether the magnitude (absolute values) was high or low, and there wasno concept of the above distribution of birefringence in the opticalmembers at all, either. For example, for measuring the birefringence ofa silica glass member, it was common recognition to those skilled in theart to measure the birefringence at several points near 95% of thediameter of the member and employ a maximum as a birefringence value inthe member. However, the inventors precisely measured the distributionof birefringence of silica glass members and found that the actualdistribution of birefringence was nonuniform.

Therefore, the inventors found out that influence of birefringence ineach member was not able to be evaluated sufficiently by simply managingthe maximum of birefringence values at several points in the member evenif the silica glass member had high uniformity of refractive index and,particularly, that it was very difficult to obtain an optical system ofdesired performance in combination of plural members. The reason whysuch nonuniform distribution of birefringence values is formed in thesilica glass member is conceivably that the nonuniform distribution ofbirefringence values is formed in the member during cooling of thesilica glass member because of temperature distribution duringsynthesis, nonuniform distribution of impurities, or nonuniformdistribution of structural defects of SiO₂.

Since the evaluation of birefringence in the overall optical systemconstructed of a plurality of optical members was not able to beexpressed simply by only the magnitude of birefringence in theindividual optical members as discussed above, the inventors preciselyinvestigated how the nonuniform distribution of birefringence values inthe optical members affected the optical system. As a result, theinventors first discovered that, with attention being focused ondirections of the fast axis as to the nonuniform distribution ofbirefringence values, the birefringence values were added up tonegatively affect the performance of the optical system when the opticalsystem was constructed of optical members having their respectivedistributions of birefringence values with the same direction of thefast axis, and that negative effects due to the birefringence of theindividual members canceled out in the overall optical system whereoptical members having different directions of the fast axis werecombined conversely, and thus accomplished the present invention.

Namely, a projection optical system of the present invention is aprojection optical system having at least two silica glass opticalmembers, wherein the optical members are combined with each other so asto satisfy such a placement condition that a signed birefringencecharacteristic value of the entire projection optical system is between−0.5 and +0.5 nm/cm both inclusive, said signed birefringencecharacteristic value being calculated in such a manner that abirefringence value is measured at each of points in a plane normal tothe optical axis with a center at an intersection with the optical axisin each optical member, a distribution of signed birefringence values ineach optical member is obtained based on a plurality of birefringencevalues and directions of the fast axis thereof, and the signedbirefringence characteristic value of the entire optical system iscalculated based on the distributions of signed birefringence values.

When the optical members are combined so as to satisfy the aboveplacement condition based on the signed birefringence values, thenonuniform distributions of birefringence values in the individualoptical members can be quantitatively evaluated with attention on thedirections of the fast axis and the optical system can be assembledwhile quantitatively estimating the signed birefringence characteristicvalue of the entire optical system from the signed birefringence valuesof the optical members so as to cancel the distributions ofbirefringence in the optical members with each other, thereby obtainingthe projection optical system with good imaging performance.

A projection exposure apparatus of the present invention is a projectionexposure apparatus comprising an exposure light source, a reticle inwhich a pattern original image is formed, an illumination optical systemfor illuminating the reticle with light emitted from the exposure lightsource, a projection optical system for projecting a pattern imageoutputted from the reticle, onto a photosensitive substrate, and analignment system for achieving alignment of the photosensitive substratewith the reticle, wherein the projection optical system is theprojection optical system of the present invention described above.

The provision of the projection optical system of the present inventionpermits the projection exposure apparatus of the present invention toattain excellent resolution.

Further, a production method of the projection optical system accordingto the present invention is a production method of a projection opticalsystem having at least two silica glass optical members, the productionmethod comprising a step of measuring a birefringence value at each ofpoints in a plane normal to the optical axis with a center at anintersection with the optical axis in each optical member and obtaininga distribution of signed birefringence values in the plane normal to theoptical axis, based on a plurality of birefringence values anddirections of the fast axis thereof, a step of calculating a signedbirefringence characteristic value of the entire projection opticalsystem, based on the distributions of signed birefringence values of therespective optical members, and a step of combining the optical memberswith each other so as to satisfy such a placement condition that thesigned birefringence characteristic value of the entire projectionoptical system is between −0.5 and +0.5 nm/cm both inclusive.

The concept of “signed birefringence value” in the present inventionwill be described below.

The signed birefringence value means a birefringence value provided witha sign in consideration of the direction of the fast axis defined in theindex ellipsoid in the measurement of birefringence values of an opticalmember.

More specifically, in the plane normal to the optical axis with thecenter at the intersection between the optical member and the opticalaxis, an area under circular irradiation of light is defined as a nearlycircular, effective section, the plus (or minus) sign is assigned to abirefringence value measured when the direction of the fast axis in asmall area at a birefringence measuring point on this effective sectionis parallel to a radial direction from the center at the intersectingpoint between the optical member and the optical axis, and the minus (orplus) sign is assigned when perpendicular.

The above sign assigning method to the birefringence values can also beapplied to cases wherein a plurality of beams are radiated into theplane normal to the optical axis with the center at the intersectingpoint between the optical member and the optical axis. In such cases,the plus (or minus) sign is assigned to a birefringence value measuredwhen the direction of the fast axis in a small area at a birefringencemeasuring point on an effective section of each of the areas underillumination with the plurality of beams is parallel to a radialdirection from the center at the intersection between the optical memberand the optical axis, and the minus (or plus) sign is assigned whenperpendicular.

Further, the above sign assigning method to the birefringence values canalso be applied to cases wherein the beams have the shape other than thecircular cross section in the plane normal to the optical axis with thecenter at the intersection between the optical member and the opticalaxis, for example, to cases of beams of a ring cross section or anelliptic cross section. In these cases, the plus (or minus) sign isassigned to a birefringence value measured when the direction of thefast axis in a small area at a birefringence measuring point on aneffective section of each of the areas under illumination with theplurality of beams is parallel to a radial direction from the center atthe intersection between the optical member and the optical axis, andthe minus (or plus) sign is assigned when perpendicular.

The following will describe the cases wherein the plus sign is assignedto a birefringence value measured when the direction of the fast axis inthe small area at the birefringence measuring point on the effectivearea under irradiation with the beam is parallel to a radial directionfrom the center at the intersection between the optical member and theoptical axis and the minus sign is assigned when perpendicular.

The signed birefringence value will be described below in further detailwith reference to FIG. 1A, FIG. 1B, FIG. 2A, FIG. 2B, FIG. 3A, and FIG.3B.

FIG. 1A is a schematic diagram to show directions of the fast axis atbirefringence measuring points P₁₁, P₁₂, P₁₃, and P₁₄ the distance r₁,r₂, r₃, or r₄, respectively, apart from the center O₁, on the effectivesection of the optical member L1. In this figure the birefringencemeasuring points P₁₁ to P₁₄ are positioned on a straight line Q₁extending radially through the center O₁ for convenience's sake ofdescription. In the FIG., the size of a small area indicated by a circleat each measuring point corresponds to an optical path difference ateach measuring point. Directions of segments W₁₁, W₁₂, W₁₃, and W₁₄ inthese small areas indicate the directions of the fast axis. Since allthe directions of the fast axis at the measuring points P₁₁ to P₁₄ areparallel to the direction of the straight line Q₁, i.e., to the radialdirection, all the birefringence values at the measuring points P₁₁ toP₁₄ are expressed with the plus sign. When these signed birefringencevalues A₁₁, A₁₂, A₁₃, A₁₄ at the measuring points P₁₁, to P₁₄illustrated in FIG. 1A, obtained as described above, are plotted againstthe distance in the radial direction, the distribution obtained is, forexample, the profile as illustrated in FIG. 1B.

FIG. 2A is a schematic diagram to show directions of the fast axis atbirefringence measuring points P₂₁, P₂₂, P₂₃, P₂₄ the distance r₁, r₂,r₃, or r₄, respectively, apart from the center O₂ on the effectivesection of the optical member L2, similar to FIG. 1A. In this case,since all the directions of the fast axis W₂₁, W₂₂, W₂₃, W₂₄ at themeasuring points P₂₁ to P₂₄ are normal to the direction of the straightline Q₂, i.e., to the radial direction, all the signed birefringencevalues A₂₁, A₂₂, A₂₃, A₂₄ at the measuring points P₂₁ to P₂₄ areexpressed with the minus sign. When these signed birefringence valuesA₂₁ to A₂₄ at the measuring points P₂₁ to P₂₄ illustrated in FIG. 2A,obtained as described above, are plotted against the distance in theradial direction, the distribution obtained is, for example, the profileas illustrated in FIG. 2B.

FIG. 3A is a schematic diagram to show directions of the fast axis atbirefringence measuring points P₃₁, P₃₂, P₃₃, P₃₄, and P₃₅ the distancer₁, r₂, r₃, r₄, or r₅, respectively, apart from the center O₃ on theeffective section of the optical member L3, similar to FIG. 1A. In thiscase, the directions of the fast axis W₃₁, W₃₂, W₃₃, W₃₄, and W₃₅ at themeasuring points P₃₁ to P₃₅ are such that those at the measuring pointsP₃₁ to P₃₃ are parallel to the direction of the straight line Q₃, i.e.,to the radial direction and those at the measuring points P₃₄, P₃₅ areperpendicular to the radial direction, and thus the distribution of thesigned birefringence values A₃₁ to A₃₅ at the measuring points P₃₁ toP₃₅ against the distance in the radial direction is the profile asillustrated in FIG. 3B.

Next, the “signed birefringence characteristic value of the entireprojection optical system” in the present invention will be describedbelow based on FIG. 4A and FIG. 4B.

FIG. 4A is a schematic side view in which m optical members constitutingthe projection optical system are arranged in order from the lightsource. FIG. 4B is a schematic, cross-sectional view to show theeffective section normal to the optical axis, of the optical member Lilocated at the ith position from the light source out of the m opticalmembers illustrated in FIG. 4A.

In the present invention, it is assumed that the distribution ofbirefringence values in the optical member is uniform in the directionof the thickness of the member parallel to the optical-axis directionbut nonuniform in the radial direction on the effective section normalto the optical axis. Here the “effective section” means an area underirradiation of light in the plane normal to the optical axis of theoptical member. An intersection of the effective section with theoptical axis is defined as a center of the effective section and theradius thereof as an effective radius of the effective section of theoptical member. In the measurement of the signed birefringencecharacteristic value of the entire projection optical system, since thesizes of the effective sections are different among the optical members,the sizes of the effective sections of all the optical members arepreliminarily normalized so that the maximum effective radius r_(n) ofeach optical member becomes one as illustrated in FIG. 4A.

When a plurality of beams are radiated into the plane normal to theoptical axis with the center at the intersection between the opticalmember and the optical axis, the sizes of the effective sections of allthe optical members are preliminarily normalized so that the maximumeffective radius r_(n) of each optical member becomes one for each ofthe effective sections corresponding to the individual beams.

Further, in the cases wherein beams having the shape other than thecircular cross section, for example, beams of the ring section or theelliptic section are radiated into the plane normal to the optical axiswith the center at the intersection between the optical member and theoptical axis, the sizes of the effective sections of all the opticalmembers are also preliminarily normalized so that the maximum effectiveradius r_(n) of each optical member becomes one for each of theeffective sections corresponding to the individual beams.

For example, when the beams of the ring section are radiated, the sizesof the effective sections of all the optical members are preliminarilynormalized so that the maximum outside radius of the ring becomes one,and the measurement of signed birefringence values can be performed in amanner similar to the measurement with the beams of the circular crosssection described hereinafter. When the beams of the elliptic sectionare radiated, the sizes of the effective sections of all the opticalmembers are preliminarily normalized so that the maximum outside lengthof the major axis of the ellipse becomes one, and the measurement ofsigned birefringence values can be carried out in a manner similar tothe measurement with the beams of the circular section described below.

For measuring the signed birefringence characteristic value of theentire projection optical system, a first step is to establish ahypothetical model of concentric circles C_(ij) with the center Oi andwith their respective radii from the center on the effective section forone optical member Li, as illustrated in FIG. 4B. Then a birefringencevalue is measured at the kth measuring point P_(ijk) on the jthconcentric circle C_(ij) with the radius of r_(j) from the center O_(i).Further, the sign is assigned to the measurement from the relationbetween the direction of the fast axis and the radial direction at themeasuring point P_(ijk) to obtain the signed birefringence value A_(ijk)at the measuring point P_(ijk).

Here the character “i” represents the numbers (i=1, 2, . . . , m; 2≦m)of the optical members L forming the projection optical system. Further,the character “j” represents the numbers (j=1, 2, . . . , n; 1≦n) of theconcentric circles C with the center on the optical axis and with theirrespective radii from the optical axis different from each other,hypothetically given on the effective section normal to the optical axisin the optical member L. Further, the character “k” represents thenumbers (k=1, 2, . . . , h; 1≦h) of measuring points on thecircumference of the concentric circles C. In this way the signedbirefringence values A_(ijl) to A_(ijh) are measured at thepredetermined measuring points P_(ijl) to P_(ijh) on each singleconcentric circle C_(ij).

Then an average signed birefringence value B_(ij), which is anarithmetic mean of the signed birefringence values at the measuringpoints on the circumference of the concentric circle C_(ij) in theoptical member Li, is calculated according to the equation below.$\begin{matrix}{B_{ij} = \frac{\sum\limits_{k = 1}^{h}\quad A_{ijk}}{h}} & (6)\end{matrix}$

Then E_(ij), which indicates an average signed birefringence amount asthe product of the average signed birefringence value B_(ij) and theapparent thickness T_(i), is calculated according to the equation below.

E _(ij) =B _(ij) ×T _(i)  (5)

In this equation T_(i) represents the apparent thickness of the opticalmember Li. This apparent thickness is either one properly selected froman average of thicknesses in the effective section of the optical memberLi and an effective thickness based on matching with other members aboveand below the optical member Li when placed in the optical system.

Then an average change amount G_(j) of signed birefringence values,which is a result of division of the summation of average signedbirefringence amounts E_(ij) in the entire projection optical system bythe total path length D, is calculated according to the equation below.$\begin{matrix}{G_{j} = \frac{\sum\limits_{i = 1}^{m}\quad E_{ij}}{D}} & (3)\end{matrix}$

In this equation D represents an apparent total path length of theentire projection optical system indicated by the following equation.$\begin{matrix}{D = {\sum\limits_{i = 1}^{m}T_{i}}} & (4)\end{matrix}$

Then the signed birefringence characteristic value H of the entireprojection optical system, which is a result of division of thesummation of average change amounts G_(j) of the signed birefringencevalues in the entire projection optical system by the number n ofconcentric circles, is calculated according to the equation below.$\begin{matrix}{H = \frac{\sum\limits_{j = 1}^{n}\quad G_{j}}{n}} & (2)\end{matrix}$

When the signed birefringence characteristic value H of the entireprojection optical system thus calculated satisfies the followingequation, the entire projection optical system demonstrates excellentimaging performance and the projection exposure apparatus provided withsuch a projection optical system shows excellent resolution.

−0.5≦H≦+0.5 nm/cm  (1)

The present invention will be more fully understood from the detaileddescription given hereinbelow and the accompanying drawings, which aregiven by way of illustration only and are not to be considered aslimiting the present invention.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will beapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an explanatory diagram to show the concept of signedbirefringence values and FIG. 1B is a graph to show the distribution ofsigned birefringence values in the optical member illustrated in FIG.1A.

FIG. 2A is another explanatory diagram to show the concept of signedbirefringence values and FIG. 2B is a graph to show the distribution ofsigned birefringence values in the optical member illustrated in FIG.2A.

FIG. 3A is another explanatory diagram to show the concept of signedbirefringence values and FIG. 3B is a graph to show the distribution ofsigned birefringence values in the optical member illustrated in FIG.3A.

FIG. 4A is a side view to show a plurality of optical membersconstituting a projection optical system and FIG. 4B is across-sectional view of the optical member forming the projectionoptical system of FIG. 4A.

FIG. 5 is a schematic, structural diagram to show an example of theprojection optical system of the present invention.

FIG. 6 is a flowchart to show an example of the production method of theprojection optical system according to the present invention.

FIG. 7 is an explanatory diagram to illustrate a furnace for synthesisof a silica glass ingot used in the present invention.

FIG. 8A is a cross-sectional view of an optical member to show measuringpoints of signed birefringence values in the optical member forming theprojection optical system of the present invention and FIG. 8B is agraph to show a distribution of average signed birefringence values inthe optical member illustrated in FIG. 8A.

FIG. 9A is a cross-sectional view of another optical member to showmeasuring points of signed birefringence values in the optical memberforming the projection optical system of the present invention and FIG.9B is a graph to show a distribution of average signed birefringencevalues in the optical member illustrated in FIG. 9A.

FIG. 10A is a cross-sectional view of another optical member to showmeasuring points of signed birefringence values in the optical memberforming the projection optical system of the present invention and FIG.10B is a graph to show a distribution of average signed birefringencevalues in the optical member illustrated in FIG. 10A.

FIG. 11 is a schematic, structural diagram to show an example of theprojection exposure apparatus of the present invention.

FIG. 12A and FIG. 12B are explanatory diagrams to show an example of thestructure of the illumination optical system in the projection exposureapparatus illustrated in FIG. 11.

FIG. 13 is a schematic diagram to show a silica glass ingot produced asa primary material for optical members forming the examples andcomparative examples of the projection optical system of the presentinvention.

FIGS. 14A to 14D are graphs to show distributions of average signedbirefringence values in optical members forming the examples andcomparative examples of the projection optical system of the presentinvention.

FIGS. 15A to 15F are graphs to show distributions of average signedbirefringence values in optical members used in Examples 1 to 3 and inComparative Examples 1 to 3 of the projection optical system of thepresent invention.

FIG. 16A is a schematic, structural diagram to show an example of theconventional projection exposure apparatus and FIG. 16B is across-sectional view of the fly's eye lens used in the projectionexposure apparatus of FIG. 16A.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, the projection optical system of the present invention will bedescribed. FIG. 5 is a schematic, structural diagram to show an exampleof the projection optical system of the present invention.

The projection optical system 100 illustrated in FIG. 5 is composed of afirst lens unit G1 of a positive power, a second lens unit G2 of apositive power, a third lens unit G3 of a negative power, a fourth lensunit G4 of a positive power, a fifth lens unit G5 of a negative power,and a sixth lens unit G6 of a positive power, which are arranged in theorder named from the side of reticle R as a first object. The opticalsystem is approximately telecentric on the object side (reticle R side)and on the image side (wafer W side) and has a reduction ratio. Thisprojection optical system has N.A. of 0.6 and the projectionmagnification of ¼.

In this projection optical system, a single crystal of calcium fluorideis used at six positions of L45, L46, L63, L65, L66, and L67 for thepurpose of correcting for chromatic aberration.

The above projection optical system of the present invention is designedin such a manner that the signed birefringence characteristic value ofthe entire projection optical system is calculated according to theaforementioned algorithms of Eqs. (1) to (6) from the distribution ofsigned birefringence values in the plane normal to the optical axis AXwith the center at the intersection with the optical axis AX for each ofthe optical members L11 to L610 and that the optical members arecombined with each other so as to satisfy the placement condition thatthis signed birefringence characteristic value of the entire projectionoptical system is between −0.5 and +0.5 nm/cm both inclusive.

When the optical members are combined so as to satisfy theaforementioned placement condition based on the signed birefringencevalues, the nonuniform distribution of birefringence values in each ofthe individual optical members can be quantitatively evaluated withattention on directions of the fast axis and the optical system can beassembled while quantitatively estimating the signed birefringencecharacteristic value of the entire optical system from the signedbirefringence values of the respective members so as to cancel thedistributions of birefringence values of the optical members with eachother, whereby the projection optical system can be obtained withexcellent imaging performance.

In the projection optical system of the present invention, it ispreferable that the optical members be combined with each other so as tofurther satisfy such a placement condition that a Strehl value of signedbirefringence values based on effective paths of the entire projectionoptical system is not less than 0.93.

The inventors discovered that use of Strehl intensities of signedbirefringence values taking account of the effective paths in the centerand surroundings thereof in the effective section of each optical memberwas effective to the evaluation of the birefringence distribution in theoptical members. Since the Strehl value of birefringence firstintroduced by the inventors takes account of the effective paths of rayspassing through the effective sections, it allows more preciseevaluation of birefringence distribution in the optical members whencombined with the evaluation with the signed birefringencecharacteristic value of the entire optical system.

This placement condition of each optical member based on the Strehlvalue of signed birefringence values is expressed based on the followingequations.

 0.93≦S  (7)

$\begin{matrix}{S = {\prod\limits_{i = 1}^{m}\quad S_{i}}} & (8) \\{S_{i} = {1 - {\left( \frac{2\pi}{\lambda} \right)^{2} \cdot \left( {\frac{\sigma^{2}}{2} + \frac{{X}^{2}}{4}} \right)}}} & (9)\end{matrix}$

[In Eqs. (7) to (9), λ indicates the wavelength of the light source. Xrepresents an average of signed birefringence values determined from thedistribution in an effective radial direction of signed birefringencevalues based on the effective optical paths obtained for the opticalmember Li by ray tracing of the entire projection optical system. σrepresents the standard deviation of signed birefringence valuesdetermined from the distribution in the effective radial direction ofsigned birefringence values based on the effective paths obtained forthe optical member Li by ray tracing of the entire projection opticalsystem. S_(i) indicates the Strehl intensity of signed birefringencevalues based on the effective paths for each optical member Li. Srepresents the Strehl value of signed birefringence values based on theeffective paths of the entire projection optical system in combinationof all the optical members Li.]

In the projection optical system of the present invention, it is furtherpreferable that the signed birefringence values around the center O_(i)of the optical member Li be not more than 0.2 nm/cm. Since most of thelight projected onto the optical members has the optical axis in thecentral part of the optical members, use of the optical memberssatisfying the above condition can greatly decrease the influence ofbirefringence as compared with cases of use of optical members havingbirefringence in the central area.

In the projection optical system of the present invention, it is alsopreferable that the distribution in the radial direction of averagesigned birefringence values B_(ij) in the optical member Li have noextremum except at the center O_(i). Further, where the distribution ofsigned birefringence values of the optical member has no extremum exceptat the center, it is easy to estimate the signed birefringencecharacteristic value of the entire optical system and desired opticalperformance can be achieved by effectively canceling the influence ofbirefringence of the individual members with each other.

In the projection optical system of the present invention, it is furtherpreferable that a difference ΔB_(i) between a maximum and a minimum ofthe average signed birefringence values B_(ij) in the optical member Libe not more than 2.0 nm/cm.

The inventors discovered that excellent imaging performance of theprojection optical system was able to be attained by constructing theprojection optical system of such optical members. It was further foundthat the projection exposure apparatus provided with the projectionoptical system was able to realize uniform resolution throughout theentire wafer surface. The larger the difference ΔB_(i) between maximumand minimum in the radial distribution of average signed birefringencevalues B_(ij), the larger the dispersion in the average signedbirefringence values B_(ij) of the optical member, in turn, thedispersion in the signed birefringence values A_(ijk). When light isprojected onto an optical member in which the difference ΔB_(i) betweenmaximum and minimum of average signed birefringence values B_(ij) islarger than 2.0 nm/cm, the wavefront of the light is disturbed becauseof the large difference of signed birefringence values A_(ijk) dependingupon passing locations of light and it tends to extremely lower theimaging performance of the optical system.

In the projection optical system of the present invention, it is alsopreferable that a maximum F_(i) of slope of a distribution curve in theradial direction of average signed birefringence values B_(ij) in eachoptical member Li be not more than 0.2 nm/cm per 10 mm of radial width.Excellent imaging performance of the projection optical system can beattained by constructing the projection optical system of such opticalmembers. Further, when the projection exposure apparatus is providedwith such a projection optical system, the resolution becomes uniformthroughout the entire wafer surface. Just as in the case of the largedifference ΔB_(i) between maximum and minimum in the radial distributionof the average signed birefringence values B_(ij) as described above,the larger the maximum F_(i) of slope of the distribution curve in theradial direction of the average signed birefringence values B_(ij), thelarger the dispersion in the average signed birefringence values B_(ij)of the optical member, in turn, the dispersion in the signedbirefringence values A_(ijk). When light is projected onto an opticalmember in which the maximum F_(i) of slope of the distribution curve inthe radial direction of the average signed birefringence values B_(ij)greater than 0.2 nm/cm per 10 mm of radial width, the wavefront of lightis disturbed because of the large difference of signed birefringencevalues A_(ijk) depending upon passing locations of light and it tends togreatly lower the imaging performance of the optical system.

Although the conventional “magnitude of birefringence (birefringencevalue)” and the “difference between maximum and minimum of signedbirefringence values” in the present invention both are expressed inunits of nm/cm, the former is a value indicated by the maximum |max| ofbirefringence values without consideration to the directions of the fastaxis, whereas the latter is indicated by the difference between amaximum value in a portion where the direction of the fast axis isparallel to the radial direction of the optical member (the maximum ofplus-signed birefringence values) and a maximum value in a portion wherethe direction of the fast axis is perpendicular to the radial directionof the optical member (the maximum of minus-signed birefringencevalues): (+A_(max))-(−A_(max)); or (+B_(max))-(−B_(max)) in the case ofthe average signed birefringence values B_(ij).

Next, the production method of the projection optical system accordingto the present invention will be described below.

FIG. 6 is a flowchart to show an example of the production method of theprojection optical system of the present invention.

As illustrated, the production method of the projection optical systemof the present invention is mainly comprised of step 200 of producing asynthetic silica glass ingot or ingots, step 210 of cutting opticalmembers from the synthetic silica glass ingot(s) obtained, step 220 ofcarrying out a thermal treatment of the optical members thus cut, step230 of measuring the signed birefringence values A_(ijk) in the opticalmembers after the thermal treatment, step 240 of calculating the averagesigned birefringence values B_(ijk) and the average signed birefringencevalues E_(ij) from the signed birefringence values A_(ijk) obtained,step 250 of selecting optical members to be used, from the distributiondata of signed birefringence values in the optical members obtained instep 240, step 260 of arranging a plurality of optical members selectedin step 250, under the placement condition based on the signedbirefringence characteristic value of the entire projection opticalsystem, step 270 of measuring the Strehl value of signed birefringencevalues based on the effective paths for the projection optical systemthus assembled in step 260, and step 280 of rearranging the opticalmembers under the placement condition based on the Strehl value obtainedin step 270.

The production method of the projection optical system of the presentinvention will be detailed below according to the flowchart of FIG. 6.

First described is the production of the synthetic silica glass in step200.

The silica glass members used in the projection optical system of thepresent invention are made by either of methods, for example, includinga) a method in which a silicon compound is subjected to hydrolysis inoxyhydrogen flame to obtain glass particles (so called soot) and inwhich the glass particles are deposited to form porous glass (so calleda soot body). The porous glass is treated at temperatures over near thesoftening point (preferably, the melting point) thereof to becometransparent, thereby obtaining transparent silica glass; b) a method inwhich a silicon compound is subjected to hydrolysis in oxyhydrogen flameto effect deposition of resultant glass particles on a target andvitrification thereof simultaneously, thus obtaining the transparentsilica glass. The method of a is called a soot method and the method ofb a direct method.

In the soot method, there are no specific restrictions on how to formthe porous glass, and it is made by one selected from a VAD method, anOVD method, a sol-gel method, and so on.

The following describes a method of producing the silica glass member bythe direct method (which is also called a flame hydrolysis process).

FIG. 7 shows a synthesis furnace 400 for synthesis of the silica glassingot 470 used in the present invention.

A burner 410 is made of silica glass and in multiple tube structure andis installed with its tip portion directed from the top of the furnacetoward the target 420. The furnace wall is composed of furnace frame 440and refractory 430 and is equipped with an observation window (notillustrated), a window 450 for monitoring with an IR camera, and anexhaust system 460. The target 420 for formation of ingot 470 is locatedin the lower part of the furnace and the target 420 is coupled through asupport shaft 480 to an XY stage (not illustrated) located outside thefurnace. The support shaft 480 can be rotated by a motor and the XYstage can be moved two-dimensionally in the X-direction and in theY-direction by an X-axis servo motor and by a Y-axis servo motor.

An oxygen-containing gas and a hydrogen-containing gas are ejected fromthe burner 410 to be mixed to create the flame. The feed of the siliconcompound diluted with a carrier gas is ejected from the central part ofthe burner into this flame whereupon the feed is decomposed byhydrolysis to form silica glass particles (soot). This soot is depositedonto the rotating and swinging target and, at the same time as it, it isfused and vitrified to obtain the ingot of transparent silica glass. Atthis time, the target is pulled down in the Z-direction so that theupper part of the ingot is covered by the flame and so that the positionof the synthetic surface in the upper part of the ingot is always keptat an equal distance from the burner.

The feed ejected from the central part of the burner 410 can be oneselected from silicon chlorides such as SiCl₄, SiHCl₃, and so on;silicon fluorides such as SiF₄, Si₂F₆, and so on; organic siliconcompounds including siloxanes such as hexamethyldisiloxane,octamethylcyclotetrasiloxane, tetramethylcyclotetrasiloxane, and so on,silanes such as methyltrimethoxysilane, tetraethoxysilane,tetramethoxysilane, and so on; SiH₄, Si₂H₆, and so on.

The simplest method of obtaining a silica glass member in which thesigned birefringence values around the center of the effective sectionof the silica glass member are between −0.2 and +0.2 nm/cm bothinclusive, is a method of producing a large-diameter ingot by theabove-stated production method of the synthetic silica glass and cuttingthe optical member of a desired diameter from the ingot. In this case,it is necessary to match the geometric center of the optical member withthat of the ingot. Since the large-diameter ingot tends to have a flatdistribution of signed birefringence values as compared with asmall-diameter ingot, a distribution of distortion also becomes flat inthe optical member cut out of the ingot. This is possibly because thetemperature gradient in the central part of the synthetic surface of theingot is smaller than the temperature gradient in the peripheral part ofthe synthetic surface close to the side surface of the ingot.

For obtaining a silica glass member with no extremum in the distributionof signed birefringence values except around the center of the effectivesection, the large-diameter ingot is also produced first, thedistribution of signed birefringence values thereof is checked, and theoptical member is cut out of the ingot so as not to have no extremumexcept in the central part. In another method, the optical member iskept at high temperatures enough to relax birefringence and thereafteris subjected to an annealing operation to gradually cool the opticalmember, whereby the silica glass member is obtained with thedistribution of signed birefringence values in which birefringence inthe central part is not more than 0.2 nm/cm and in which there is noextremum except in the central part.

The cutting method from the large-diameter ingot and the annealingoperation to relax birefringence are also effective in obtaining theoptical members with little dispersion in the distribution of signedbirefringence values inside the members wherein the difference betweenmaximum and minimum of signed birefringence values is not more than 2.0nm/cm or wherein the maximum of slope of the distribution of signedbirefringence values in the radial direction of signed birefringencevalues is not more than 0.2 nm/cm per 10 mm of width. It is alsopossible to decrease the dispersion in the signed birefringence valuesto some extent, by controlling synthesis conditions, for example, byoptimizing the temperature distribution in the upper part (synthesissurface) of the ingot during the synthesis.

The optical members are then cut in step 210 from the ingot(s) thusproduced. In step 220, further, the optical members thus cut aresubjected to the thermal treatment of quick heating→ short-timeretention→ quick cooling, thereby restraining the dispersion in thesigned birefringence values in the optical members as describedpreviously. Produced in this way are candidates for optical membershaving various shapes to construct the projection optical systemillustrated in FIG. 5. Specifically, for producing the projectionoptical system illustrated in FIG. 5, a plurality of silica glass lensesare fabricated in the same shape and in the same size for the silicaglass lens L11 illustrated in FIG. 5.

Then the signed birefringence values are measured for each of theoptical members in step 230. Namely, the signed birefringence valuesA_(ijk) are measured at a plurality of measuring points P_(ijk) on aplurality of concentric circles postulated in the effective sectionperpendicular to the optical axis of the optical members.

Measuring methods of birefringence applicable in the present inventionwill be described below. A phase modulation method will be describedfirst. An optical system is composed of a light source, a polarizer, aphase modulation element, a sample, and an analyzer. The light source isan He—Ne laser or a laser diode, and the phase modulation element is aphotoelastic converter. Light from the light source is converted intolinearly polarized light by the polarizer and the linearly polarizedlight is incident to the phase modulation element. The light from thephase modulation element to be projected onto the sample is modulatedlight with states of polarization continuously varying as linearlypolarized light→circularly polarized light→linearly polarized light bythe element. During the measurement the sample is rotated about the beamincident to a measuring point on the sample to find a peak of output ofa detector and the amplitude at that peak is measured to determine thedirection of the fast axis (or the slow axis) and the magnitude ofbirefringent phase difference. The measurement can also be performedwithout rotation of the sample by use of a Zeeman laser as the lightsource. It is also possible to employ the phase shift method and theoptical heterodyne interferometry in the present invention.

In addition, the measurement can also be conducted by the followingmethods though they are a little inferior in measurement accuracy.

A rotary analyzer method employs a device configuration in which thesample between the light source and the photodetector is interposedbetween the polarizer and the rotary analyzer. Signals from the detectorare measured while rotating the analyzer placed after the sample to bemeasured, and a phase difference is calculated between a maximum and aminimum of the signals from the detector.

In a phase compensation method there are a light source, a polarizer, asample, a phase compensator, an analyzer, and a photodetector placed.When the axes of the polarizer and analyzer are positioned in mutuallyorthogonal states, linearly polarized light incident to the sample to bemeasured is converted into elliptically polarized light because of thebirefringence of the sample. Then the phase compensator is adjusted soas to return the light to linearly polarized light. With adjustment ofthe compensator, the signal becomes almost null at the detector. A phasecompensation value at the highest extinction rate is an amount ofbirefringence.

The measurement can also be performed by a simple method of placing astandard sample in a crossed Nicols optical system and comparing thesample with the standard sample if the thickness of the measured sampleis large enough.

Each measurement of birefringence is given the sign of + when thedirection of the fast axis is parallel to the diameter of the member,but the sign of − when perpendicular, as described previously. A smallmeasurement of birefringence can have an inclination while the fast axisis not always perfectly parallel or normal to the diameter. In this casethe sign of + is assigned if the inclination is closer to parallel thanthe angle of 45° relative to the diameter, whereas the sign of − if theinclination is closer to normal.

In step 240 the average signed birefringence values B_(ijk) are thencalculated from the signed birefringence values A_(ijk) for each of theoptical members Li, obtained in step 230. Here each average signedbirefringence value B_(ijk) is an arithmetic mean of a plurality ofsigned birefringence values at a plurality of measuring points on aconcentric circle C_(ij) at an equal distance r_(j) from the centerO_(i) of the effective section.

How to calculate the average signed birefringence values B_(ijk) fromthe signed birefringence values A_(ijk) will be described belowreferring to FIG. 8A, FIG. 8B, FIG. 9A, FIG. 9B, FIG. 10A, and FIG. 10B.

FIG. 8A is a schematic diagram where the measuring points areintersections P₁₁₁, P₁₂₁, P₁₃₁, P₁₄₁, P₁₁₂, P₁₂₂, P₁₃₂, and P₁₄₂ betweenconcentric circles C₁₁, C₁₂, C₁₃, and C₁₄ having their respective radiir₁, r₂, r₃, and r₄ from the center O₁ on the effective section of theoptical member L1 and two straight lines Q101 and Q102 extending in aradial direction from the center O₁. In this case, an average signedbirefringence value on each concentric circle of the optical member L1is an arithmetic mean of signed birefringence values obtained from twomeasuring points on the circumference of a single concentric circle.More specifically, in the case of the concentric circle C₁₁, thearithmetic mean of signed birefringence values A₁₁₁, and A₁₁₂ obtainedat the measuring point P₁₁₁ and at the measuring point P₁₁₂ on thecircumference of the concentric circle C₁₁, is the average signedbirefringence value B₁₁, represents the signed birefringence values ofthe point group on the circumference of the concentric circle C₁₁. Insimilar fashion, thereafter, the average signed birefringence values B₁₂to B₁₄ are calculated for the concentric circles C₁₂ to C₁₄. Then theaverage signed birefringence values B₁₁, to B₁₄ are illustrated as afunction of distance from the center O₁, whereby the distribution ofaverage signed birefringence values in the radial direction of theoptical member L1 can be quantitatively understood. For example, wherethe average signed birefringence values B₁₁, to B₁₄ all are positivevalues monotonically increasing in the radial direction, the profile asillustrated in FIG. 8B can be obtained as the distribution of averagesigned birefringence values in the radial direction of the opticalmember L1.

FIG. 9A is a schematic diagram where the measuring points areintersections P₂₁₁, P₂₂₁, P₂₃₁, P₂₄₁, P₂₁₂, P₂₂₂, P₂₃₂, and P₂₄₂ betweenconcentric circles C₂₁, C₂₂, C₂₃, and C₂₄ having their respective radiir₁, r₂, r₃, and r₄ from the center O₂ on the effective section of theoptical member L2 and two straight lines Q₂₀₁ and Q₂₀₂ extending in aradial direction from the center O₂. In this case, the average signedbirefringence values B₂₁ to B₂₄ are also obtained for the concentriccircles C₂₁ to C₂₄, as described in the FIG. 8A. When the average signedbirefringence values B₂₁ to B₂₄ are illustrated as a function ofdistance from the center O₂, the distribution of average signedbirefringence values in the radial direction of the optical member L2can be quantitatively understood. For example, where the average signedbirefringence values B₂₁ to B₂₄ all are negative values monotonicallydecreasing in the radial direction, the profile as illustrated in FIG.9B is obtained as the distribution of average signed birefringencevalues in the radial direction of the optical member L2.

FIG. 10A is a schematic diagram where the measuring points areintersections P₃₁₁, P₃₂₁, P₃₃₁, P₃₄₁, P₃₅₁, P₃₁₂, P₃₂₂, P₃₃₂, P₃₄₂, andP₃₅₂ between concentric circles C₃₁, C₃₂, C₃₃, C₃₄, and C₃₅ having theirrespective radii r₁, r₂, r₃, r₄, and r₅ from the center O₃ on theeffective section of the optical member L3 and two straight lines Q₃₀₁and Q₃₀₂ extending in the radial direction from the center O₃. In thiscase, the average signed birefringence values B₂₁ to B₂₅ are alsoobtained for the concentric circles C₃₁ to C₃₅, as described in the FIG.8A. When the average signed birefringence values B₃₁ to B₃₅ areillustrated as a function of distance from the center O₃, thedistribution of average signed birefringence values in the radialdirection of the optical member L3 can be quantitatively understood. Forexample, supposing that the average signed birefringence values B₃₁ toB₃₅ are positive values, that B₃₄ and B₃₅ are negative values, and thatthese average signed birefringence values B₃₁ to B₃₃ take a maximumvalue near r₂ and monotonically decrease in the radial direction from r₂to r₅, the profile as illustrated in FIG. 10B can be obtained as thedistribution of average signed birefringence values in the radialdirection of the optical member L3.

Further, in step 240, the average signed birefringence amounts E_(ijk)for each of the optical members are calculated according to Eq. (5) fromthe average signed birefringence values B_(ijk) for each optical member,obtained as described above, and the apparent thickness T_(i) of eachoptical member.

Next, optical members used are selected in step 250. On this occasion,since the optical members have their respective effective sectionsdifferent from each other, depending upon their roles in the projectionoptical system, as illustrated in the projection optical system in FIG.5, the effective sections are normalized so that the maximum effectiveradius is one, for all the optical members, as illustrated in FIG. 4A.Based on the common effective radius after the normalization, thefollowing selection of optical members is carried out with reference tothe radial distributions of average signed birefringence values B_(ijk)of all the optical members, thereby narrowing down the candidates forthe optical members used at their respective positions in the projectionoptical system.

The selection conditions of optical members in this step 250 are asfollows: selection condition 251; the signed birefringence values aroundthe center O_(i) of the optical member Li are not more than 0.2 nm/cm,selection condition 252; the difference ΔB_(i) between maximum andminimum of the average signed birefringence values B_(ij) in the opticalmember Li is not more than 2.0 nm/cm, selection condition 253; themaximum F_(i) of slope of the distribution curve in the radial directionof the average signed birefringence values B_(ij) in each optical memberLi is not more than 0.2 nm/cm per 10 mm of radial width, and selectioncondition 254; the radial distribution of the average signedbirefringence values B_(ij) in the optical member Li has no extremumexcept at the center O_(i).

By using optical members to meet all or at least one of the aboveselection conditions 251 to 254, the projection optical system with highimaging performance can be constructed more efficiently.

In step 260 the optical members are then arranged so that the signedbirefringence characteristic value H of the entire projection opticalsystem, described previously by Eq. (1), becomes between −0.5 and +0.5nm/cm both inclusive. At this time, the signed birefringencecharacteristic value H of the entire projection optical system iscalculated according to Eqs. (2) to (4) described previously. Theprojection optical system arranged in this way demonstrates excellentimaging performance.

In step 270, 280 the optical members are then combined with each otherso as to further satisfy the placement condition that the Strehl valueof the signed birefringence values based on the effective paths of theentire projection optical system is not less than 0.93. Since the Strehlvalue of birefringence takes account of the effective paths of rayspassing the effective sections, more precise evaluation of birefringencedistributions in the optical members can be made when combined with theevaluation with the signed birefringence characteristic value of theentire optical system. The Strehl value S of birefringence is calculatedbased on Eq. (8) and Eq. (9), using the data of radial distributions ofsigned birefringence values obtained by ray tracing.

For example, in the case of the projection optical system 100illustrated in FIG. 5, the measurement results of radial distributionsof the signed birefringence values for the silica glass lenses L11 toL610 except for the lenses L45, L46, L63, L65, L66, and L67 made of thesingle crystal of calcium fluoride, are entered into a computer forcalculation of the Strehl value. Then ray-passing points on each lensare determined on the optical axis, in the paraxial area, off the axis,and so on by a technique according to ray tracing used in calculation ofaberration of optical systems or the like, and the Strehl value iscalculated by substituting the signed birefringence values correspondingto the passing points into Eq. (9). Namely, a plurality of Strehl valuesare calculated corresponding to a plurality of rays incident at avariety of incident angles into the projection optical system 100 and aminimum among them is determined as a Strehl value of the combination ofthe samples.

As for the distributions of signed birefringence values of the sixlenses made of the single crystal of calcium fluoride, a theoreticalvalue can be put into the computer, or an actually measured value of amaterial having a standard distribution of birefringence values can beentered. As another method, it is also possible to prepare the materialfor these six lenses at one time and evaluate them together with thesilica glass lenses.

The above production method of the projection optical system wasdescribed as an example provided with step 250 of selecting the opticalmembers, step 270 of measuring the Strehl value, and step 280 ofarranging the optical members, based on the Strehl value, but theproduction method of the projection optical system of the presentinvention is not limited to the above example form; the above threesteps all are omissible steps in the production method of the projectionoptical system of the present invention.

In the above description, a value calculated by the following method canalso be used as a guide of reference for evaluating the influence ofbirefringence of the entire projection optical system instead of theabove signed birefringence characteristic value of the entire projectionoptical system in the cases wherein the radial distribution of averagesigned birefringence values B_(ijk) is a slightly monotonicallyincreasing or slightly monotonically decreasing distribution while thebirefringence values are almost zero around the center for all theoptical members, in the measurement of the radial distribution of theaverage signed birefringence values B_(ijk) for each of the opticalmembers. Namely, it is a value calculated in such a way that anarithmetic mean is calculated from values in the radial direction of theaverage signed birefringence values B_(ijk) in each optical member to bedefined as a signed birefringence value representing the member and thatthe signed birefringence values of all the optical members are added up.The members are combined so as to make this addition result zero in theentire optical system, which allows simple selection of members takingthe influence of birefringence into consideration.

FIG. 11 is a schematic, structural diagram of an example of theprojection exposure apparatus provided with the projection opticalsystem of the present invention. In FIG. 11, the Z-direction is takenalong a direction parallel to the optical axis of the projection opticalsystem 304, the Y-direction along a direction normal to the Z-directionwithin the plane of the drawing, and the X-direction along a directionnormal to the plane of the drawing and to the Z-direction.

The projection exposure apparatus illustrated in FIG. 11 is mainlycomposed of an exposure light source 303, a reticle R in which a patternoriginal image is formed, an illumination optical system 302 forilluminating the reticle R with light emitted from the exposure lightsource 303, a projection optical system 304 for projecting the patternimage outputted from the reticle R onto the wafer (photosensitivesubstrate) W, and an alignment system 305 for achieving alignmentbetween the reticle R and the wafer W.

The wafer W is mounted on a leveling stage (not illustrated) and thisleveling stage is placed on a Z-stage 301, which can be finely moved inthe direction of the optical axis of the projection optical system(i.e., in the Z-direction) by a driving motor 320. The Z-stage 301 ismounted on an XY stage 315, which can be moved in the two-dimensional(XY) directions in the step-and-repeat method by a driving motor 330.The reticle R is mounted on a reticle stage 306, which can be movedtwo-dimensionally in the horizontal plane. The exposure light from theexposure light source 303 uniformly illuminates the pattern formed inthe reticle R through the illumination optical system 302 whereupon thepattern image of the reticle R is transferred into a shot area of thewafer W by the projection optical system 304. This exposure light can beone having the wavelength selected from 248 nm (KrF excimer laser), 193nm (ArF excimer laser), 157 nm (F₂ laser), and so on.

After completion of projection of the pattern of the reticle R into oneshot area on the wafer W, the XY stage 315 undergoes such steppingmovement that a next shot area of the wafer W is aligned with theexposure area of the projection optical system 304. The two-dimensionalposition of the leveling stage with the wafer W mounted thereon isalways monitored, for example, in the resolution of about 0.01 μm bymeasuring the distance to a moving mirror 340 fixed to the levelingstage, with a laser interferometer (not illustrated), and output of thelaser interferometer is supplied to a stage control system 311.

The reticle R is positioned on the reticle stage 306 so that the centerof the transferred pattern on the reticle R is aligned with the opticalaxis AX of the projection optical system 304. The positioning of thereticle R is carried out using a plurality of reticle alignment marks(reticle marks) provided near the periphery of the reticle R. Thereticle marks include two types of reticle marks, i.e., reticle marksfor positioning in the X-direction and reticle marks for positioning inthe Y-direction. The alignment system 305 uses the exposure light, whichis extracted by splitting part of the exposure light from the exposurelight source 303, as illumination light (alignment light). The alignmentsystem 305 includes alignment units located one at the position of eachreticle alignment mark.

The illumination light having passed through the illumination opticalsystem 302 is then incident to the reticle marks provided outside thepattern area of the reticle R. The reticle marks are, for example,rectangular transparent windows formed in an opaque region around thepattern. The alignment light reflected from the reticle mark portions isincident again into the alignment system 305. On the other hand, thealignment light having passed through the reticle marks travels throughthe projection optical system 304 to be incident onto substratealignment marks (wafer marks) provided in the periphery of each shotarea on the wafer W. The wafer marks do not always have to be providedin the peripheral area of each shot area, but may also be provided atpredetermined positions of the wafer, for example, only in theperipheral region of the wafer. The wafer marks also include two typesof wafer marks, i.e., wafer marks for positioning in the X-direction andwafer marks for positioning in the Y-direction corresponding to thereticle marks. Reflected light from the wafer marks travels in pathsopposite to those of the incident light and through the projectionoptical system 304 and through the reticle mark portions, to be incidentagain into the alignment system 305.

Receiving the reflections of the alignment light from the reticle R andfrom the wafer W in this way, the alignment system 305 detects therelative positions of the reticle R and the wafer W. Output of thealignment system 305 is supplied to a main control system 312. Output ofthe main control system 312 is supplied to a reticle exchange system 307and to the stage control system 311 to adjust the spatial positions ofthe reticle R and the wafer W. As a result, registration can bemaintained at high accuracy between the pattern formed in each shot areaon the wafer W and the pattern image of the reticle R to be projectedthereto.

FIG. 12A and FIG. 12B are schematic, structural diagrams to show thedetailed structure of the illumination optical system 302 in theprojection exposure apparatus illustrated in FIG. 11.

FIG. 12A is a front view of the illumination optical system 302 whenobserved from the Y-direction of FIG. 11 and FIG. 12B is a front view ofthe illumination optical system 302 when observed from the X-directionof FIG. 11. The both figures are illustrated with omitting the alignmentsystem 305 which uses the split part of the exposure light incident tothe illumination optical system 302.

The exposure light source 303 (not illustrated) emits an almost parallelbeam having the wavelength of 248 nm (KrF excimer laser), 193 nm (ArFexcimer laser), 157 nm (F₂ laser), or the like and the cross-sectionalshape of the parallel beam at this point is rectangular. This parallelbeam from the exposure light source 303 is incident to a beam shapingoptical system 20 as a beam shaping section for shaping the beam into apredetermined cross-sectional shape. This beam shaping optical system 20is composed of two cylindrical lenses (20A, 20B) having refractive powerin the Y-direction; the cylindrical lens 20A on the light source sidehas negative refractive power to diverge the beam in the X-direction,while the cylindrical lens 20B on the illuminated surface side haspositive refractive power to condense the diverging beam from thelight-source-side cylindrical lens 20A into a parallel beam. Therefore,the parallel beam from the exposure light source 303, having passedthrough the beam shaping optical system 20, is one shaped in therectangular shape of the beam section having a predetermined size withthe beam width expanded in the Y-direction. The beam shaping opticalsystem 20 can also be constructed of a combination of cylindrical lenseshaving positive refractive power, an anamorphic prism, or the like.

The shaped beam from the beam shaping optical system 20 is incident to afirst relay optical system 21. Here the first relay optical system 21has a front unit (21A, 21B) of positive refractive power consisting oftwo positive lenses, and a rear unit (21C, 21D) of positive refractivepower consisting of two positive lenses, and they are arranged so thatthe front unit (21A, 21B) of the first relay optical system 21 forms aconverging point (light source image) I at the focal point of this frontunit on the reticle R side (rear side) and so that the rear unit (21C,21D) of the first relay optical system 21 has the focal point on thelight source side (front side), matched with the focal point of thefront unit (21A, 21B). Then this first relay optical system 21 has thefunction of keeping the exit plane of the exposure light source 303 inconjugate with the entrance plane of an optical integrator 30 as firstmultiple light source image forming means, described hereinafter. Thisfunction of the first relay optical system 21 serves to correctdeviation of the light illuminating the optical integrator 30 due toangular deviation of the light from the exposure light source 303 andincrease tolerance to the angular deviation of the light from theexposure light source 303. A guide optical system for guiding the lightfrom the exposure light source 303 to the first multiple light sourceimage forming means is comprised of the beam shaping optical system 20and the first relay optical system 21.

The beam having passed through the first relay optical system 21 is thenincident to the optical integrator 30 as the first multiple light sourceimage forming means for forming a plurality of light source imagesarranged linearly in three rows. This optical integrator 30 is composedof a plurality of lens elements of the double-convex shape having analmost square lens section, and the whole of the optical integrator 30has a rectangular cross section. Each lens element of the double-convexshape has equal curvature (refractive power) in the Y-direction and inthe X-direction.

For this reason, each of parallel beams passing through the individuallens elements forming the optical integrator 30 is condensed to form alight source image on the exit side of each lens element. Therefore, aplurality of light source images are formed in the number equal to thenumber of lens elements and at the exit position A1 of the opticalintegrator 30, substantially creating secondary light sources there.

Beams from the plurality of secondary light sources created by theoptical integrator 30 are condensed by a second relay optical system 40to be further incident to an optical integrator 50 as second multiplelight source image forming means for forming a plurality of light sourceimages.

This optical integrator 50 is composed of a plurality of lens elementsof the double-convex shape having a rectangular lens section and thecross-sectional shape of this lens element is similar to that of theoptical integrator 30. The whole of the optical integrator 50 has asquare cross section. Each of the lens elements has equal curvature(refractive power) in the direction on the plane of FIG. 12a and in thedirection on the plane of FIG. 12b.

For this reason, each of beams from the optical integrator 30 travelingthrough the individual lens elements forming the optical integrator 50is condensed to form a light source image on the exit side of each lenselement. Therefore, a plurality of light source images arranged in thesquare shape are formed at the exit position A2 of the opticalintegrator 50, substantially creating tertiary light sources there.

The second relay optical system 40 keeps the entrance position B1 of theoptical integrator 30 in conjugate with the entrance position B2 of theoptical integrator 50 and also keeps the exit position A1 of the opticalintegrator 30 in conjugate with the exit position A2 of the opticalintegrator 50. Further, the optical integrator 30 and the opticalintegrator 50 were described in the shape of the fly's eye lenses in theabove description, but there are no specific restrictions on the shapeof the optical integrators used in the illumination system of theprojection exposure apparatus of the present invention; for example,they can be selected from micro-fly's eyes consisting of a plurality ofextremely small lens elements, rodlike internal reflection type opticalelements (kaleidoscope rods), diffracting optical elements (DOE), and soon.

An aperture stop AS having an aperture of a predetermined shape islocated at the position A2 where the tertiary light sources are formed,or at a position near it, and beams from the tertiary light sources,formed in a circular shape by this aperture stop AS, are condensed by acondenser optical system 60 as a condensing optical system to uniformlyilluminate the reticle R as an object to be illuminated, in a slitshape.

The projection optical system 304 illustrated in FIG. 11 is one in whichthe optical members are combined with each other so as to satisfy theplacement condition that the signed birefringence characteristic valueof the entire projection optical system is between −0.5 and +0.5 nm/cmboth inclusive. The optical members are also combined with each other soas to further satisfy the placement condition that the Strehl value ofthe signed birefringence values based on the effective paths of theentire projection optical system is not less than 0.93. Further, theoptical members used are those satisfying the condition that the signedbirefringence values around the center of the effective section arebetween −0.2 and +0.2 nm/cm both inclusive, the condition that theradial distribution of average signed birefringence values has noextremum except at the center, the condition that the difference ΔB_(i)between maximum and minimum of average signed birefringence values isnot more than 2.0 nm/cm, and the condition that the maximum F_(i) ofslope of the distribution curve in the radial direction of the averagesigned birefringence values B_(ij) is not more than 0.2 nm/cm per 10 mmof radial width.

Since the influence of birefringence in the optical members can becontrolled in the minimum level by the provision of the projectionoptical system of the present invention, the projection exposureapparatus can be constructed with high resolution.

The projection optical system of the present invention, the productionmethod thereof, and the projection exposure apparatus using it will bedescribed below in detail with examples.

Production of Synthetic Silica Glass

First described is a method for producing the silica glass members usedin Example 1 to Example 3 and in Comparative Example 1 to ComparativeExample 3 which will be described below.

Using the synthesis furnace illustrated in FIG. 7, the synthetic silicaglass was prepared by the direct method (flame hydrolysis method).First, silicon tetrachloride was ejected from the central part of themulti-tube burner to be subjected to hydrolysis in the oxyhydrogenflame, thereby obtaining the glass particles. The particles werevitrified while being deposited onto the rotating and swinging target,to obtain the silica glass ingot 470 of φ400 mm×t800 mm illustrated inFIG. 13. The swinging width (stroke) in the X-axis direction at thistime was set to 100 mm in the first synthesis and to 150 mm in thesecond synthesis.

In each of the syntheses, the ingot 470 was made to cool in this shapeand thereafter test pieces e, m, and f were cut out in the cylindricalshape of φ400 mm×t100 mm from the ingot. Specifically, the test piece ewas cut from the portion at the heights of 500 to 600 mm from the bottomsurface of the ingot 470 obtained in the first synthesis, and the testpiece m was from the portion at the heights of 500 to 600 mm from thebottom surface of the ingot 472 obtained in the second synthesis. Thetest piece f was cut from the portion at the heights of 100 to 200 mmfrom the bottom surface of the ingot 470 obtained in the first synthesisor the ingot 472 obtained in the second synthesis. These test pieces e,m, f were heated up to the temperature of 1000° C., kept thereat for tenhours, and thereafter cooled at the rate of 10° C./hour down to 500° C.After that, they were again made to cool, thereby yielding the thermallytreated test pieces e, m, f.

On the other hand, silica glass ingots of φ120 mm to φ300 mm wereprepared by the production method of the synthetic silica glass similarto the above method except that the swinging width (stroke) in theX-axis direction was set to 50 mm. Test pieces were cut in appropriatesize from these ingots and the thermal treatment was conducted underconditions similar to those above. Then test pieces g of φ100 mm to φ280mm, necessary for the optical system of FIG. 5, were cut out.

Evaluation of Synthetic Silica Glass

The radial distribution of signed birefringence values was measured foreach of the disk-shaped test pieces e, m, f, and g of the syntheticsilica glass. The measurement of birefringence was carried out by thephase modulation method. The measuring points were intersections betweena plurality of concentric circles postulated on the effective section ofeach of the test pieces e, m, f, g and two straight lines extending inthe radial direction from the center of the effective section, asillustrated in FIG. 10A.

First, values of the signed birefringence values after cut in desireddiameters were estimated from the data of the radial distributions ofsigned birefringence values for the test pieces e, m, f, g and theselection of usable test pieces was conducted so as to minimize thesigned birefringence values of the entire optical system by combiningmembers having the plus-signed birefringence values with members havingthe minus-signed birefringence values.

The sizes of the members necessary for assembly of the projectionoptical system 100 illustrated in FIG. 5 are φ100 to φ280. It was foundthat in this range the test pieces e, m, f except for the test pieces ghad the signed birefringence values which monotonically increased ordecreased in the radial direction and the absolute values of which wererelatively small, 2 nm/cm.

Then an average was calculated of the radial distribution of signedbirefringence values for each of the test pieces e, m, f to obtain atentative typical value of the signed birefringence values for each ofthe test pieces. Then the optical system was assembled by selecting andcombining the test pieces e, m, f, typical values thereof were added upin the entire optical system, and it was confirmed that the additionresult of the entire optical system was ±0. In this way the selection ofmembers was first conducted in the simple manner with consideration tothe influence of birefringence and then the test pieces e, m, f withgood characteristics were cut more accurately out of the glass ingots.The method of evaluating the birefringent characteristics in the membersby use of the tentative typical values of the signed birefringencevalues as described above is just a guide and the aforementioned signedbirefringent characteristic value of the entire projection opticalsystem will be calculated again on the occasion of actual assembly ofthe projection optical system.

Based on the above selection, the test pieces e, m, f of silica glasswere again cut out of the ingots so that the center position of eachtest piece was aligned with a rotationally symmetric center position ofeach ingot determined from the ingot outside diameter. In practice, themembers of the desired sizes were cut so that the center position ofeach test piece was matched with the center of the thermal treatment ofeach thermally treated or annealed test piece. FIGS. 14A to 14C presentradial distribution curves of average signed birefringence valuesB_(ijk) in test pieces of φ200 mm among the test pieces e, m, f of thecylindrical shape thus cut. Each of these synthetic silica glasses has amonotonically increasing or monotonically decreasing pattern of averagesigned birefringence values B_(ijk). The signed birefringence valuesA_(io) around the center of each test piece e, m, f are all between −0.2and +0.2 nm/cm both inclusive. It was also verified that each of thetest pieces e, m, f had no extremum of average signed birefringencevalues B_(ijk) except at the center. Further, it was verified that themaximum F_(i) of slope of the radial distribution curve of averagesigned birefringence values B_(ijk) was not more than 0.2 nm/cm per 10mm and that the difference ΔB_(i) between maximum and minimum in theradial distribution of average signed birefringence values B_(ijk) wasnot more than 2.0 nm/cm.

On the other hand, FIG. 14D presents a radial distribution curve ofaverage signed birefringence values B_(ij) in a test piece of φ200 mmamong the test pieces g of the cylindrical shape cut out. It was foundthat the test piece g had a positive maximum at the position between 70%and 80% in the radius except around the center in the radialdistribution of average signed birefringence values B_(ij).

A plurality of pieces were produced for each of the above test pieces e,m, f, g in order to actually assemble the projection optical systemusing them. Specifically, a plurality of silica glass ingots were firstprepared under the same conditions as in the synthesis method describedpreviously and a plurality of pieces were produced for each of the abovetest pieces e, m, f, g so that their radial distribution characteristicsof the average signed birefringence values B_(ij) of the test pieces cutout of the plurality of silica glass ingots thus formed were matchedwith those of the above test pieces e, m, f, g.

EXAMPLE 1

The above test pieces e, m, f were processed into the respective lensshapes and the projection optical system 100 illustrated in FIG. 5 wasassembled. More specifically, the twenty three lenses except for thelenses L45, L46, L63, L65, L66, and L67 made of the calcium fluoridecrystal out of the optical members constituting the projection opticalsystem 100 were made of the test pieces e, m, and f. Then the signedbirefringence characteristic value H of the entire projection opticalsystem and the Strehl value S of signed birefringence values werecalculated for the complete projection optical system, and the system inplacement demonstrating the best values was defined as Example 1.Further, the resolution was measured where Example 1 was used as theprojection optical system of the projection exposure apparatusillustrated in FIG. 11. FIG. 15A shows the radial distributions ofaverage signed birefringence values B_(ij) in a combination of one eachof the test pieces e,

EXAMPLE 2

The projection optical system was assembled in the same manner as inExample 1 except that the test pieces e, f were used. Further, theresolution was measured where Example 2 was applied to the projectionoptical system of the projection exposure apparatus illustrated in FIG.11. FIG. 15B shows the radial distributions of average signedbirefringence values B_(ij) in a combination of one each of the testpieces e, f.

EXAMPLE 3

The projection optical system was assembled in the same manner as inExample 1 except that the test pieces m, f, and g were used. Further,the resolution was measured where Example 3 was applied to theprojection optical system of the projection exposure apparatusillustrated in FIG. 11. FIG. 15C shows the radial distributions ofaverage signed birefringence values B_(ij) in a combination of one eachof the test pieces m, f, g.

COMPARATIVE EXAMPLE 1

The projection optical system was assembled in the same manner as inExample 1 except that the test piece g was used. Further, the resolutionwas measured where Comparative Example 1 was applied to the projectionoptical system of the projection exposure apparatus illustrated in FIG.11. FIG. 15D shows the radial distribution of average signedbirefringence values B_(ij) of the test piece g.

Comparative Example 2

The projection optical system was assembled in the same manner as inExample 1 except that the test pieces f, g were used. Further, theresolution was measured where Comparative Example 2 was applied to theprojection optical system of the projection exposure apparatusillustrated in FIG. 11. FIG. 15E shows the radial distributions ofaverage signed birefringence values B_(ij) in a combination of one eachof the test pieces f, g.

Comparative Example 3

The projection optical system was assembled in the same manner as inExample 1 except that the test piece f was used. Further, the resolutionwas measured where Comparative Example 3 was applied to the projectionoptical system of the projection exposure apparatus illustrated in FIG.11. FIG. 15F shows the radial distribution of average signedbirefringence values B_(ij) of the test piece g.

Table 1 presents the optical characteristics based on the signedbirefringence of the projection optical systems in Examples 1 to 3 andin Comparative Examples 1 to 3 described above and the measured valuesof resolution of the projection exposure apparatus using them.

SILICA EXTREMUM OF GLASS Aio Bij EXCEPT AT ΔBi Fi H USED nm/cm CENTER Oinm/cm nm/cm nm/cm S RESOLUTION Ex. 1 e,m,f e : ≈ 0 e : NO e : 1.6 e : ≦0.2 0.12 0.99 0.15 m : ≈ 0 m : NO m : 1.6 m : ≦ 0.2 f : ≈ 0 f : NO f :0.5 f : ≦ 0.1 Ex. 2 e,f e : ≈ 0 e : NO e : 1.6 e : ≦ 0.2 0.17 0.97 0.18f : ≈ 0 f : NO f : 1.6 f : ≦ 0.1 Ex. 3 m,f,g m : ≈ 0 m : NO m : 1.6 m :≦ 0.2 0.38 0.94 0.18 f : ≈ 0 f : NO f : 1.6 f : ≦ 0.1 g : ≈ 0 g :PRESENT g : 3.5 g : ≦ 0.4 Cmpr. 1 g g : ≈ 0 g : PRESENT g : 3.5 g : ≦0.4 1.9 0.85 UNMEASURABLE Cmpr. 2 f,g f : ≈ 0 f : NO f : 1.6 f : ≦ 0.10.6 0.92 0.20 g : ≈ 0 g : PRESENT g : 3.5 g : ≦ 0.4 Cmpr. 3 f f : ≈ 0 f: NO f : 1.6 f : ≦ 0.1 0.93 0.90 0.22

The results of the optical characteristics of Examples 1 to 3 in Table 1verified that the excellent imaging performance was demonstrated by theprojection optical systems of the present invention satisfying theplacement condition based on the signed birefringence values, i.e., thecondition that the signed birefringence characteristic value of theentire projection optical system was between −0.5 and +0.5 nm/cm bothinclusive. It was also verified that the very high resolution wasachieved where the projection optical systems of the present inventionwere used as the projection optical system of the projection exposureapparatus. Particularly, the projection exposure apparatus employingExample 1 attained the high resolution of 0.15.

The excellent imaging performance was also achieved in Example 3 partlyusing the test piece g with great dispersion in the radial distributionof average signed birefringence values B_(ij) and the projectionexposure apparatus using it demonstrated the good value of resolution,which verified the validity of the production method of the projectionoptical system of the present invention in which the optical memberswere arranged so as to keep the signed birefringence characteristicvalue between −0.5 and +0.5 nm/cm both inclusive with paying attentionto the distributions of signed birefringence values in the opticalmembers.

On the other hand, the projection optical systems in Comparative Example1 to Comparative Example 3 had the signed birefringence characteristicvalues of not less than +0.5 nm/cm and failed to demonstrate the goodimaging performance and the projection exposure apparatus using themfailed to yield as good values of resolution as in Examples 1 to 3.Particularly, in Comparative Example 1 the wavefront aberration of theentire projection optical system was far over the measurable range andit was thus impossible to measure the resolution.

As described above, the present invention allows us to quantitativelyevaluate the nonuniform distributions of birefringence values in theoptical members with attention on directions of the fast axis andfurther allows us to assemble the optical systems while quantitativelyestimating the signed birefringence characteristic value of the entireoptical system from the signed birefringence values of the respectiveoptical members so as to cancel out the distributions of birefringencein the optical members. Therefore, the invention permits minimization ofthe influence from the nonuniform distributions of birefringence valuesin the optical members on the imaging performance of the projectionoptical system or on the resolution of the projection exposure apparatusand thus provides the projection optical system with high imagingperformance, the production method thereof, and the projection exposureapparatus capable of achieving the high resolution.

From the invention thus described, it will be obvious that the inventionmay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedfor inclusion within the scope of the following claims

What is claimed is:
 1. A projection optical system comprising: at least one of a first optical member made of a silica glass and at least one of a second optical member made of a single crystal of calcium fluoride, wherein said at least one first optical member and said at least one second optical member are combined with each other so as to satisfy a placement condition such that a signed birefringence characteristic value of the entire projection optical system is in the range from −0.5 to +0.5 nm/cm, wherein said signed birefringence characteristic value of the entire projection optical system is calculated such that: a birefringence value is measured at a variety of points in a plane normal to an optical axis with a center at an intersection of each of said first and second optical members with the optical axis; and a distribution of signed birefringence values in each of said first and second optical members is obtained based on the birefringence values measured and a direction of a fast axis at each of the measured points, and wherein the signed birefringence characteristic value is calculated based on the distributions of signed birefringence values.
 2. A projection exposure apparatus comprising: an exposure light source; a reticle in which a pattern is formed; an illumination optical system that illuminates said reticle with a light emitted from said exposure light source; a projection optical system that projects an image of the pattern from said reticle onto a photosensitive substrate; and an alignment system that achieves alignment between said reticle and said photosensitive substrate, wherein said projection optical system is the projection optical system as set forth in claim
 1. 3. The projection exposure apparatus according to claim 2, wherein said exposure light source emits a light of a wavelength of not more than about 250 nm as an exposure light.
 4. A projection optical system comprising: at least one of a first optical member made of a silica glass and at least one of a second optical member made of a single crystal of calcium fluoride, wherein said at least one first optical member and said at least one second optical member are combined with each other so as to satisfy a placement condition such that a signed birefringence characteristic value of the entire projection optical system is in the range from −0.5 to +0.5 nm/cm, wherein said signed birefringence characteristic value of the entire projection optical system is calculated such that: a birefringence value of each of said first optical members is measured at a variety of points in a plane normal to an optical axis with a center at an intersection of each of said first optical members with the optical axis, a distribution of signed birefringence values in each of said first optical members is obtained based on the birefringence values of said at least one first optical member and a direction of a fast axis at each of the measured points; and a theoretical distribution of signed birefringence values in each of said second optical members is obtained in the plane normal to the optical axis with a center at an intersection of each of said second optical members with the optical axis, and wherein the signed birefringence characteristic value is calculated based on the distributions of signed birefringence values and the theoretical distributions of signed birefringence values.
 5. A projection exposure apparatus comprising: an exposure light source; a reticle in which a pattern is formed; an illumination optical system that illuminates said reticle with a light emitted from said exposure light source; a projection optical system that projects an image of the pattern from said reticle onto a photosensitive substrate; and an alignment system that achieves alignment between said reticle and said photosensitive substrate, wherein said projection optical system is the projection optical system as set forth in claim
 4. 6. The projection exposure apparatus according to claim 5, wherein said exposure light source emits a light of a wavelength of not more than about 250 nm as an exposure light.
 7. A projection optical system comprising: at least one of a first optical member made of a silica glass and at least one of a second optical member made of a single crystal of calcium fluoride, wherein said at least one first optical member and said at least one second optical member are combined with each other so as to satisfy a placement condition such that a Strehl value of at least one signed birefringence value is based on at least one effective path of the entire projection optical system is not less than about 0.93, wherein said Strehl value is calculated such that a birefringence value is measured at a variety of points in a plane normal to an optical axis with a center at an intersection of each of said first and second optical members with the optical axis, wherein a distribution of signed birefringence values in each of said first and second optical members based on the effective path of each of said first and second optical members is obtained based on the birefringence values and a direction of a fast axis at each of the measured points, and wherein the Strehl value is calculated based on the distributions of signed birefringence values.
 8. A projection exposure apparatus comprising: an exposure light source; a reticle in which a pattern is formed; an illumination optical system that illuminates said reticle with a light emitted from said exposure light source; a projection optical system that projects an image of the pattern from said reticle onto a photosensitive substrate; and an alignment system that achieves alignment between said reticle and said photosensitive substrate, wherein said projection optical system is the projection optical system as set forth in claim
 7. 9. The projection exposure apparatus according to claim 8, wherein said exposure light source emits a light of a wavelength of not more than about 250 nm as an exposure light.
 10. A projection optical system comprising: at least one of a first optical member made of a silica glass and at least one of a second optical member made of a single crystal of calcium fluoride, wherein said at least one first optical member and said at least one second optical member are combined with each other so as to satisfy a placement condition such that a Strehl value of at least one signed birefringence value based on at least one effective path of the entire projection optical system is not less than about 0.93, wherein said Strehl value is calculated such that a birefringence value of each of said first optical members is measured at a variety of points in a plane normal to an optical axis with a center at an intersection of each of said first optical members with the optical axis, a distribution of signed birefringence values in each of said first optical members based on the effective path of each of said first optical members is obtained based on the birefringence values of said at least one first optical member and a direction of a fast axis at each of the measured points, wherein a theoretical distribution of signed birefringence values in each of said second optical members is obtained in a plane normal to the optical axis with a center at an intersection of each said second optical members with the optical axis, and wherein the Strehl value is calculated from the distributions of signed birefringence values and the theoretical distributions of signed birefringence values.
 11. A projection exposure apparatus comprising: an exposure light source; a reticle in which a pattern is formed; an illumination optical system that illuminates said reticle with a light emitted from said exposure light source; a projection optical system that projects an image of the pattern from said reticle onto a photosensitive substrate; and an alignment system that achieves alignment between said reticle and said photosensitive substrate, wherein said projection optical system is the projection optical system as set forth in claim
 10. 12. The projection exposure apparatus according to claim 11, wherein said exposure light source emits a light of a wavelength of not more than about 250 nm as an exposure light. 